1. Field of the Invention
The present invention relates to satellite systems that facilitate position determination, and in particular to a system and method that allows satellites from GPS and GLONASS satellite systems to simultaneously provide real-time position determination.
2. Related Art
GPS (global positioning system) is radio-based satellite system developed and operated by the United States. GPS became fully operational in 1995. To provide global coverage, GPS uses between 24-32 satellites. Assuming the minimum number of 24 satellites, 4 satellites are deployed in each of six orbits. The six orbital planes' ascending nodes are separated by 60 degrees. In this configuration, a minimum of six satellites should be in view from any given point at any time.
All GPS satellites transmit at 1575 MHz, wherein a receiver can distinguish signals from different satellites because each signal is encoded with a high-rate pseudo-random (PRN) sequence of 1023 “chips” that are unique for each satellite. These chips, also called a coarse acquisition (CA) code, are continuously repeated to allow the search engines of the receiver to identify the satellites available for position determination. For example, GPS has a chipping rate of 1023 chips/ms. A CA phase, which refers to the position within the repeating CA code, can also be used to determine the satellites in view. Depending on the error correction in the receiver, GPS can provide a positioning accuracy between 1 cm and 15 meters.
GLONASS (global navigation satellite system) is a radio-based satellite system developed by the former Soviet Union and now operated by Russia. As of 2009, GLONASS has global coverage. To provide this global coverage, GLONASS includes 24 satellites, wherein 21 satellites can be used for transmitting signals and 3 satellites can be used as spares. The 24 satellites are deployed in three orbits, each orbit having 8 satellites. The three orbital planes' ascending nodes are separated by 120 degrees. In this configuration, a minimum of five satellites should be in view from any given point at any time.
All GLONASS satellites transmit the same standard precision (SP) signal, but with each satellite transmitting on a different frequency. Specifically, GLONASS uses a 15 channel FDMA (frequency division multiple access) centered on 1602.0 MHz. Therefore, each satellite transmits at 1602 MHz+(N×0.5625 MHz), wherein N is a frequency channel number (N=−7, −6, −5, . . . 5, 6). GLONASS has a chipping rate of 511 chips/ms. GLONASS can provide a horizontal positioning accuracy within 5-10 meters, and a vertical positioning accuracy within 15 meters.
With the advent of GLONASS satellites now being available to provide position information, it is desirable to have a system that includes the capability of using both GPS and GLONASS signals for position determination. Current systems include separate receive paths for each type of signal. For example, U.S. Publication 2007/0096980, filed by Gradincic et al. on Oct. 30, 2006 (and published on May 3, 2007), describes an RF receiver including a plurality of independent signal paths, each signal path including a separate IF and baseband down-converter. Each signal path is tuned to a specific IF band by selection of external IF filters.
Therefore, a need arises for a system and method for receiving both GPS and GLONASS signals that minimizes components while allowing both sets of signals to be used in the position determination, thereby improving position accuracy.